1. Field of the Invention
The present invention relates to a compatible optical pickup, and more particularly, to a compatible optical pickup which can be compatibly used for three types of recording media by using light beams with different wavelengths.
2. Description of the Related Art
An optical recording and/or reproducing apparatus records information on and/or reproduces information from an optical information storage medium, such as an optical disc, by using a light spot condensed by an objective lens. The recording capacity of the optical disc is determined by a size of the light spot. A size S of a light spot is determined by a wavelength λ of the light and a numerical aperture (NA) of the objective lens as represented in Equation 1:S∝λ/NA  (1)Accordingly, in order to reduce the size of the light spot focused on an optical disc and to increase the density of the optical disc, a short wavelength light source (such as a blue violet laser) and an objective lens (such as an objective lens with an NA of more than 0.6) are required.
Also, when θ denotes a tilt angle of the optical disc, n denotes a refractive index of the optical disc, and d denotes the thickness of the optical disc, a coma aberration W31 caused by the tilt of the optical disc can be represented as in Equation 2:
                              W          31                =                              -                          d              2                                ⁢                                                                      n                  2                                ⁡                                  (                                                            n                      2                                        -                    1                                    )                                            ⁢              sin              ⁢                                                          ⁢              θcos              ⁢                                                          ⁢              θ                                                      (                                                      n                    2                                    -                                                            sin                      2                                        ⁢                                                                                  ⁢                    θ                                                  )                                            5                /                2                                              ⁢                      NA            3                                              (        2        )            wherein the refractive index n and the thickness d of the optical disc refer to a refractive index and the thickness of an optical medium ranging from a light incidence surface to a recording surface.
As can be seen in Equation 2, in order to secure a tolerance by the tilt of the optical disc, the thickness of the optical disc must be reduced with an increase in the NA of the objective lens for high density. For example, a compact disc (CD) has the thickness of 1.2 mm, and the thickness of a digital versatile disc (DVD) is reduced to 0.6 mm. A next generation DVD with recording capacity of more than 20 GB, which is currently in progress of standardization and development and stores high definition (HD) moving picture information, is highly likely to be manufactured to the thickness of 0.1 mm. Of course, for the CD, the NA of the objective lens is 0.45, and for the DVD, the NA of the objective lens is increased to 0.6. However, for the next generation DVD, the NA of the objective lens is highly likely to increase to more than 0.6, and may be for example, 0.85. Also, in view of recording capacity, there is a high probability that a blue violet light source to emit blue violet light with a wavelength of about 405 nm will be used for the next generation DVD. When developing an optical information storage medium according to new standards, the compatibility with an existing optical information storage medium is problematic.
For example, since a reflectance of a writable DVD (DVD-R) and a writable CD (CD-R) is considerably reduced according to the wavelength, the use of a light source to emit light with wavelengths of 650 nm and 780 nm is indispensable. Accordingly, in terms of the compatibility with the DVD-R and the CD-R, an optical pickup used for the next generation DVD may adopt three light sources which emit light beams with different wavelengths.